Deprecate setting slots with custom tag and attributes

thinking about this once again, I don't think there is any good reason to deprecate this. Yes, this is somewhat unneeded API since the same can be achieved by creating a custom element and passing it as slot, but that's not as straight forward to do when you just want to create a slot with some html markup.

I think we can still support this, the code is there, it's not complicating things, so there is no reason to introduce a breaking change here really.